Avoid using back roads if possible. Use the highway as you will have less traffic light and less chance of stopping
Pump your brakes. In most places, pumping your brakes is a signal to the driver behind you that they are too close to your vehicle. This usually causes the person behind you to slow down.
Speed up if you feel that it is safe to do so AND it's within the speed limit. Sometimes, motorists get frustrated about a vehicle travelling far below the speed limit. If it's safe and legal, feel free to speed up. If you are uncomfortable at high speeds, see the next step.
Move to an outside lane (not the shoulder) and allow faster traffic to pass. In most places, faster traffic moves in the inner lanes (the "fast lanes") and slower traffic in the outer lanes (the "slow lanes"). Moving to this lane will also give you a chance to pull off onto the shoulder (if one is available) in case you need to pull out of traffic completely.
Find alternate routes. If you notice that the route you take to get wherever you're going is a constant source for tailgating or other road rage, it might be safer to find another way to go, not to mention much less stressful.
If you are on a two-lane road where passing is permissible and you're already going as fast as you are comfortable going, slow down and encourage the tail gator to pass. Waving someone forward is generally considered acceptable on country roads. [SEE WARNING BELOW]
If someone is really tailgating you and you feel unsafe, take the first right turn you can. Resume your route when the impatient motorist has passed.